Help please. I can’t solve this

Question

You have 120 feet of fencing to enclose a rectangular plot that borders on a river. If you do not fence the side along the river, find the length and width of the plot that will maximize the area

Answer 1

So you only have three sides. Call them x, x and y.

The perimeter will be x+x+y = 2x+y = 120.

The area will be A=xy

Now using your constraint, 2x+y =120, solve for x or y and plug it into the A=xy formula. You will have a parabola equation for A (or if you prefer, you will have a quadratic eq for A). Find the maximum for A using any method you are comfortable with.